The mammalian immune system is largely regulated by soluble proteins termed cytokines. In particular, the cytokines interleukin (IL)-2 and IL-15 perform critical functions in the immune system to regulate activation and homeostasis of T cells, as well as to direct development of several important lineages of immune effector cells. The signaling portion of the IL-2 receptor is also used by IL-15; therefore these cytokines elicit very similar responses in target cells. The goal of this project is to explore mechanisms by which the IL-2/15 receptor (IL-2/15R) delivers specific signals to the immune system using in vitro and in vivo models. In particular, this work will focus on signals that are delivered by a "tyrosine-deficient" IL-2/l5R. Most known signaling cascades are initiated by inducible phosphorylation of receptor tyrosine residues following cytokine binding. These phosphotylated amino acids subsequently "recruit" signaling intermediates that specifically recognize phosphotyrosine. Although phosphorylation plays a key role in signal transduction, another type of signaling in the IL-2/l5 system has recently been discovered that proceeds in the absence of tyrosines on a critical IL-2/15R subunit (the IL-2/15Rb chain). This signaling cascade serves to protect T cells from cell death (apoptosis) induced by IL-2 starvation. This effect is relevant in vivo, since IL-2 and IL-15 control survival and death of activated and memory T cells. The long term goal of this research is to define the molecular mechanisms and biological significance of the signals that derive from a tyrosine-deficient IL-2/l5R. First, this research will define the specific molecular regions within the IL-2/15R required for anti-apoptotic signaling (Aim 1). Second, this work will identify signaling intermediates in the anti-apoptotic signaling cascade by isolating and characterizing proteins that associate with the tyrosine-deficient IL-2/l5R (Aim 2). Third, this work will determine the functional significance of this form of signaling in vivo by creating mice that contain targeted tyrosine-to-phenylalanine deletions within the IL-2/l5Rb gene, and assessing consequences to the immune system (Aim 3). The IL-2/IL-15 receptor system has numerous important clinical associations. Genetic defects in genes that make up the IL-2 receptor have been found to cause immunodeficiency syndromes and autoimmunity. Indeed, exciting new findings from human gene therapy trials have demonstrated effective immune restoration, in SCID patients deficient in the IL-2Rg chain. In addition, cytokines are being used with increasing frequency to treat cancer and HIV, and IL-15 and IL-2 are effective adjuvants for certain vaccines. Finally, many immunosuppressive drugs involve IL-2 production and function. However, treatments involving cytokines often cause severe side effects. Thus, it would be very valuable clinically to develop pharmacological agents that selectively interfere with the signaling pathways that lead to side effects or selectively mimic beneficial cvtokine signaling pathways. A better understanding of the cytokine signaling response at a molecular and physiological level will facilitate the development of therapeutic tools for conditions involving cytokine function, including cancer, autoimmune disease and immune deficiency.